In transmission systems for communications, of which the telephone switching system is an example, there is a need for gain control. Gain control is accomplished in the line circuit associated with a subscriber or a trunk and enables the system user to optimize the line circuit according to a subscriber's preferences or requirements.
Basically, a subscriber's opinion of the grade of transmission in a communications system depends upon many factors, such as the received acoustic pressure, which is a function of the efficiency of the transmitter and receiver and of the electrical loss between them, the frequency response of the network as to the bandwidth, and amplitude and delay distortion, the noise as to the amount of noise and the character of the noise, the crosstalk, the echo as to magnitude and delay. In general, the transmission system should be flexible and adapted to provide the user with the most efficient and reliable transmission and reception that can be accommodated within the bounds dictated by technology and in conformity with his expectations.
In this respect, gain control is a desirable feature to be used in a line circuit as it enables greater flexibility of operation to enable a subscriber to be accommodated according to such expectations. Gain control may be employed for level adjustment in a line circuit, as well as affording improvement, by proper adjustment, of certain types of interferences. Thus, undesirable effects arising from echoes, and "singing" can be improved or reduced by the use of gain control in the line circuit. Such effects are usually caused or arise partly from impedance mismatching and gain control offers an effective improvement.
It is, of course, understood that the design and implementation of a line circuit is a prime consideration in the design of a telephone switching system as in a communications transmission system. Each subscriber in a telephone system has a separate line circuit and hence, any additional circuitry in a line circuit significantly increases the cost of the entire system. As one can imagine, a single switching system may have, for example, 100,000 subscribers and hence, 100,000 line circuits, as compared to common control components as the network control, the switching matrix and so on.
While implementation of gain control is extremely desirable in a line circuit, it must be accommodated as inexpensively as possible and of course, must provide the above noted advantages. Gain control can be employed and implemented in both the analog and digital paths of a switching system and techniques for both are known. In the system to be described, the subscriber transmits and receives an analog signal but the signal processed by the line circuit for transmission and reception via the switching matrix is a digital signal. If one implemented gain control on the analog signal, one would substantially increase the number of line circuit components. Furthermore, analog gain control requires matched, high quality precision components which are expensive. If the components are not matched, one does not achieve fine gain resolution (0.1 db or better) and therefore, many of the advantages of gain control are not provided.
It is thus an object of the present invention to provide gain control in a telephone line circuit, which control operates a on digital signal. The gain control apparatus to be described requires inexpensive components as compared to analog techniques, while affording high accuracy and resolution over an effective control range.
The digital implementation to be described allows the components used to be implemented with integrated circuit technology and hence, the gain control and other components can be fabricated in integrated circuit boards which can accommodate large amounts of digital hardware.